230 ORIGIN AND NATURE OF LIFE 



task-work, for in some instances the obvious 

 function may be destroyed without causing 

 death, its place being undertaken vicariously 

 by some other part, but interference with the 

 hobby usually leads to sudden and remarkable 

 death of the whole animal. This will be 

 reverted to later when the other main systems 

 have been mentioned. 



The amount of blood flowing to various 

 parts is carefully regulated by a set of nerves, 

 which acting upon circularly arranged muscle- 

 fibres control the diameter, or calibre of the 

 small vessels in a manner much more perfect 

 than the taps on a town water-supply. Not 

 only must food be borne to the cells, oxygen 

 must also be carried, else the cells could 

 not set the energy of the food free. The 

 waste products of the combustion must also 

 be carried away. This office is accomplished 

 by the respiratory system, which carries the 

 gaseous products to and from the lungs in 

 solution or in union with the red colouring 

 pigment of the blood and the proteins, while 

 other solid waste products are removed in 

 solution by the renal excretory system. 



Excess of fluid containing waste products, 

 lying between the tissue cells and other 

 constituents, is drained off by a system of 

 fine vessels called the lymphatics, and after 



